Silicon carbide powders are not sinterable when they are in a pure form. Previously, boron or boron carbide and carbon were added in order to densify silicon carbide. Usually, the beta phase of silicon carbide is used as the starting material, with transformation into the alpha phase occurring as the densification process continues. Regardless of the starting material, the resulting ceramic has low fracture toughness values of approximately 4 MPa .sqroot.m. This low fracture toughness value for monolithic silicon carbide limits its usage.
It is known that crack propagation in silicon carbide can be minimized by the inclusion of a dispersed phase, which provides a mechanism for energy diffusion. For example, J. J. Petrovic and R. E. Honnell, "MoSi.sub.2 Particle Reinforced-SiC and Si.sub.3 N.sub.4 Matrix Composites", J. Mat. Sci. Lett. 9 (1990) 1083-1084, disclose mixing SiC and molybdenum disilicide to improve the high temperature mechanical properties of the SiC composite. However, the molybdenum disilicide particles inhibit densification to less than 90% and the dispersion of the molybdenum disilicide is poor with large agglomerations visible in the microstructure. U.S. Pat. No. 4,729,972 (Kodoma et al.) discloses a SiC matrix containing carbide phases of titanium, zirconium, hafnium, vanadium, niobium, tantalum and tungsten having an average particle size of 30-150 microns. The sintered product has high strength and toughness.